Typically, a radio baseband LSI (Large Scale Integration) generates channel estimate values based on reference signals, and performs equalization using the channel estimate values to cancel the distortion occurring in a radio transmission channel, thereby reducing transmission errors.
Recently, an OFDM-based radio baseband standard has been extensively used because of its high transmitting efficiency. However, in the OFDM-based radio baseband standard, it is often the case that reference signals used for channel estimation are discretely arranged in frequency domain. In this case, the channel estimate values can be calculated only discretely.
Therefore, in a case of calculating a channel estimate value with respect to a frequency component in which the reference signal is not arranged, an interpolation is generally performed using its adjacent channel estimate values. Representative examples of methods of the interpolation include a linear interpolation. However, in a case of interpolating in the frequency domain, there is a problem that a difference between a channel estimate value obtained by the linear interpolation and an actual channel estimate value becomes larger.
In order to address such a problem, there is sometimes used a technique referred to as an FFT (Fast Fourier Transform) interpolation. As shown in FIG. 7, in the FFT interpolation, a delay profile is generated by performing an inverse-FFT operation for the discretely calculated channel estimate values (Step S1), zeros are added to the delay profile by numbers equivalent to all of interpolation points (Step S2), and interpolated channel estimate values are calculated by performing an FFT operation for a result of the addition (Step S3).
Meanwhile, in general cases, subcarrier signals used in the OFDM system do not exist over all of bands where the FFT operation is performed. Similarly, the reference signals used for the channel estimation do not exist over all of the bands where the FFT operation is performed, but actually exist only within a certain bandwidth among all of the bands. Therefore, as shown at Step S4 in FIG. 7, it is necessary to generate a virtual waveform of channel estimate values outside the bandwidth by an extrapolation or the like.
Examples of the extrapolation applied upon generation of the virtual waveform include a method using a simple linear interpolation. However, there is a problem that a difference between the virtual waveform and an actual waveform becomes larger.
As examples of techniques to address such a problem, PTL 1 discloses a method of extracting only pass locations from the delay profile generated by the inverse-FFT operation, performing an FFT operation for a result of the extraction, and generating the virtual waveform by adjusting locations of discontinuous points in a result of the FFT operation.
Further, PTL 2 has proposed a method of generating two tangential lines from sub carriers within the bandwidth toward the outside of the bandwidth, and generating the virtual waveform by multiplying the tangential lines by a window function.